In the field of photonic or optoelectronic integrated components, it is becoming necessary to form an increasingly large number of layers having different compositions or doping types on a substrate. Manufacturing such components thus generally requires numerous successive epitaxial growth operations and etching operations.
The molecular beam epitaxy method is well suited in particular to growing large numbers of very thin layers. In this technique, the layers are grown in a very high vacuum, generally lying in the range 10.sup.-2 Pa to 10.sup.-10 Pa, and referred to as an "ultra-high vacuum", in which the elements constituting the layer to be deposited, optionally accompanied by doping impurities, are simultaneously evaporated. Currently, the etching operations are performed outside the epitaxy chamber. Once the substrate has been removed from the chamber, a tried and tested etching method is applied to it such as liquid-phase chemical etching, or "dry etching": reactive ion etching or reactive ion beam etching. However, liquid-phase chemical etching suffers from the drawback of not being uniform, and therefore not being suitable for manufacturing wafers having large surface areas. Dry etching makes it possible to etch semiconductor components with good control of etching depth, and good uniformness on large substrates. However, it suffers from a certain number of limitations:
on the etched surface, a residual layer remains that has been rendered amorphous, and that has a thickness lying in the range 10 nanometers to 100 nanometers;
it is necessary to remove the amorphous layer by means of "wet" chemistry;
it is necessary to remove the polymerization products formed during etching; and
it is necessary to bring the etched chips back out into the air before epitaxy is resumed.
It is therefore desirable to be able to perform etching operations in the reactor serving to perform molecular beam epitaxy, while remaining under ultra-high vacuum conditions, so as to prevent any impurities from being deposited, which would require an additional preparation step to be performed.
For substrates made of III-V type semiconductor alloys such as Inp and GaAs, ultra-high vacuum etching trials have been performed by means of chemical beams of phosphorus trichloride PCl.sub.3 or of arsenic trichloride AsCl.sub.3, while raising the temperature of the substrate to in the range 500.degree. C. to 540.degree. C. Etching is obtained by the III-V compound chemically reacting with the trichloride to form reaction products that are volatile at that temperature. Unfortunately, that ultra-high vacuum chemical approach does not make it possible to remove certain dopants or impurities such as silicon, carbon, oxygen or sulfur, which do not react with the chlorine compounds. The progressive accumulation of such dopants or impurities on the surface during the etching interferes with said etching and gives rise to a high degree of roughness on the etched surface that is incompatible with good epitaxy resumption quality.
It should be noted that the problem mentioned above in a specific context arises more generally whenever a substrate or epitaxial layers having complex compositions are to be etched by means of molecular beams. In contrast, this problem is less noticeable with the liquid-phase chemical etching method because of the rinsing effect produced by that method on the surface of the substrate. Similarly, ion beam etching produces a mechanical ion-bombardment effect which makes it possible to remove the impurities that do not react chemically.
A possible solution for solving the above problem consists in performing a surface preparation step, prior to epitaxy, by means of an agent chosen so as to react with the impurities while producing volatile compounds. But such an agent must not react with the other elements in the substrate. In order to remove metalloid-type impurities such as silicon, carbon, oxygen, or sulfur, a beam could be used composed of a reducing substance such as atomic hydrogen. However, that solution is not satisfactory because the reducing substance generally reacts with other elements in the substrate that are not to be removed. When the substrate is made of a III-V semiconductor alloy such as InP, atomic hydrogen reacts with phosphorus if the temperature of the substrate is greater than 300.degree. C. Unfortunately, when the substrate is made of the InP alloy, the epitaxy operations and the PCl.sub.3 etching operations must be performed at a substrate temperature in the vicinity of 500.degree. C. The above-considered preparation therefore makes it difficult to chain between etching and resuming epitaxy.